Critical Appraisal of Pavement Design of

Ohio Department of Transportation

(ODOT)

Presented by

Pranamesh Chakraborty

Sharath M N

Indian Institute of Technology,

Kanpur

13 April 2013

Basis of Pavement Design Manual

The ODOT method for pavement design is

almost identical to the American Association

of State Highway and Transportation Officials

(AASHTO) Guide for Design of Pavement

Structures (1993).

Basic factors in pavement design

1. Serviceability/Pavement performance

expressed in terms of Present Serviceability

Rating (PSR).

2. Subgrade Soil Characterization

(expressed in terms of Subgrade Resilient

Modulus (MR)).

3. Traffic (expressed in terms of Equivalent

Single Axle Load ESAL).

4. Reliability

5. Drainage

Approach in pavement design

There are three basic approach for Pavement design

1. Empirical Approach

2. Mechanistic Approach

3. Mechanistic –Empirical Approach

ODOT/AASHTO method is an empirical

method based on the AASHO Road Test from

the late 1950s .

ODOT method

The ODOT design method is a regression relationship

between

1. # of load cycles

2. Pavement Structural capacity

3. Performance (measured in terms of serviceability)

Disadvantages of regression methods

Limitation of Application

Can be applied to conditions similar to those

for which they were developed.

Serviceability

The concept of serviceability is supported by four

fundamental assumptions:

1. Highways are for the comfort of the travelling

user;

2. The user’s opinion as to how a highway should

perform is highly subjective;

3. There are characteristics that can be measured and

related to user’s perception of performance;

4. Performance may be expressed by the mean

opinion of all users;

Structural Cracking, faulting, raveling, etc.

Functional Riding comfort (measured in terms of

roughness of pavement.)

Serviceability Performance: Measured by PSI Present

Serviceability Index with scale 0 to 5.

0 "Road closed"

5 "Just constructed" Initial PSI (pi) [4.2 (rigid)

and 4.5(flexible)]

Terminal PSI (pt)

2.5 to 3.0 for major highways

2.0 for lower class highways

1.5 for very special cases

PSI

Serviceability (contd.)

Serviceability (contd.)

Serviceability cannot be directly measured in the field.

Panel of users required to provide subjective assessments of serviceability

known as Present Serviceability Ratio (PSR).

The correlation of PSR with measured distresses is the Present Serviceability

Index (PSI).

However, PSI is the input parameter of the design equation, not the PSR,

because determining PSR is very subjective.

Alternative approaches are available correlating PSI with roughness, (Al-

Omari and Darter, 1994; Gulen et al., 1994) which is a more reliable, and

more easily measured parameter than the recommended distresses like mean

rut depth, cracking, patching, etc.

Traffic calculation

Traffic is considered in terms of ESAL.

The first step in calculating ESALs for mixed traffic is to

establish first the load equivalent factor (LEF) of every axle

of the traffic distribution.

The LEFs consider the following variables:

Axle load

Axle configuration (e.g., single, tandem, etc.)

Structural number (for flexible pavements)

Terminal serviceability

LEFs were developed based on empirical data obtained from

the AASHO Road Test.

Traffic calculation (contd.)With LEF calculated for every load group, the second step is to compute the

truck factor Tf as follows:Tf = Ʃ(pi LEFi ) A

in which:

pi = percentage of repetitions for ith load group

LEFi = LEF for the ith load group (e.g., single-12kip, tandem-22kip, etc.)

A = average number of axles per truck

The number of ESALs is calculated as follows:

ESAL = AADT T Tf G D L 365 Y

in which:

D= trucks in design direction (%)

L = trucks in design lane (%)

AADT = annual average daily traffic

T = percentage of trucks

G = growth factor

D = trucks in lane (%)

Y = design period

Discussion on Traffic calculation

It relies on a single value to represent the overall traffic

spectrum which is questionable.

The LEFs consider serviceability as the damage equivalency

between two axles.

Zhang et al. (2000) have found that LEFs determined by this,

is inconsistent with capturing damage in terms of equivalent

deflection, which is easier to measure and validate.

However quantifying damage equivalency in terms of

serviceability or even deflections is not enough to represent the

complex failure modes of pavements.

Reliability

R=p(Napp<nf)

Considers the expected traffic to be normally

distributed

R=f(ZoSo)

Reliability level required is dependent on importance

of the road

Overall standard deviation=0.39

Reliability (contd.)

f(W18)=Z0S0+g(D)

Z0 is non positive

Reliability Standard normal

deviation, Zo

50 0

60 -0.253

70 -0.524

80 -0.841

95 -1.645

99 -2.327

99.99 -3.750

Rigid Pavement Design

Design Parameters

Modulus of Rupture 700 psi

Modulus of Elasticity 5,000,000 psi

Drainage coefficient 1

Overall standard deviation 0.39

PSI

Load Transfer Coefficient

Composite Modulus of Subgrade Reaction

Loss of Support

Effective Modulus of Subgrade Reaction

Minimum thickness = 8’’

Joints

Transverse Joint

Joint spacing=21’

18’’ long Dowels

Longitudinal Joints

Mandatory when width >18’

Pressure relief joint

Composite Pavement Design

Designed as rigid pavement

Concrete thickness obtained is reduced by an inch and

replaced by 3 inches of asphalt layer

Composite Pavement Design

Designed as rigid pavement

Concrete thickness obtained is reduced by an inch and

replaced by 3 inches of asphalt layer

Design of Flexible Pavement

Design of Flexible Pavement

Design of Flexible Pavement (contd.)

An example showing computation of Structural Number

Source: AASHTO 1993

Design of Flexible Pavement (contd.)

Once SN value is set, thickness design begins…

33322211 mDamDaDaSN

where a1, a2 and a3 are structural number coefficients obtained from

nomographs for MR values of materials used.

m2 and m3 are drainage coefficients obtained from table in design

manual..

The depth that results in a SN value close to the SN value obtained

from traffic loading, etc. is the design thickness. Thus , the design

solution is not unique.

Compulsory Aggregate Base Layer

Regardless of SN required, the aggregate base layer is

to be provided rather than asphalt –on –subgrade

buildup, particularly when full depth flexible design is

very thin.

Design of Flexible Pavement (contd.)

The aggregate base is less sensitive to moisture

than the subgrade and it separates the pavement

further from the subgrade.

Use of layer coefficients a1, a2 and a3

The approach of use of layer coefficients has been found

to be inappropriate for design purposes [Coree and

White (1990)]

The layer coefficient has been found to be NOT a simple

function of the individual layer modulus, but a function

of all layer thicknesses and properties. [Baladi and

Thomas (1994)]

Improvements in Flexible Pavement Design Equation

As per the present provisions, all distresses were lumped

to one composite index- PSI.

However, by predicting individual distresses and roughness

separately, a flexible pavement design process can be

optimized to meet the specific needs.

Generate separate designs for each of the individual

distresses and an optimum can be selected such that each

of the distresses can fall below a specific level.

References AASHTO. (1993). AASHTO Guide for Design of Pavements

Structures, American Association of State Highway and Transportation Officials, Washington, DC

Al Omari, Bashar and Daughter, M. I, (992). "Relationships between IRI and PSR", Transportation engineering series no.69, University of Illinois, Urbana.

Baladi, G. Y., and A. Thomas. (1994). "Mechanistic Evaluation of AASHTO Flexible Pavement Design Equations," Transportation Research Record 1449, Washington, DC, pp. 72-78.

Coree, B. J., and T. D. White. (1990). "AASHTO Flexible Pavement Design Method: Fact or Fiction," Transportation Research Record 1286, Washington, DC, pp. 206-216.

HRB. (1962). "The AASHO Road Test. Report 7 - Summary Report," Highway Research Board, National Academy of Sciences - National Research Council, Washington, DC.

"Pavement Design Manual", Ohio department of transportation, Ohio, USA

Schwartz, W.C. and Carvalho, L.R. (2007). "Evaluation of Mechanistic-Empirical Design Procedure ", MDSHA project, Maryland.

Zhang, Z., J. P. Leidy, I. Kawa, and W. R. Hudson. (2000). "Impact of Changing Traffic Characteristics and Environmental Conditions on Flexible Pavements," Transportation Research Record 1730, Washington, DC, pp. 125-131.

Thank you all